Have you been to counseling?  I had those same feelings 4 years in.  I went to counseling as a last ditch effort to avoid putting my 2 bios through the upheaval of another divorce.  It changed my life.

Best wishes to you, it's all so tough in this life.

Is the Skid an adult?  a minor child?  are there relevent issues.. special needs?  toxic EX... ?  What exactly is causing the stress.. poor behavior by your SK? Conflict with your SO? 

How does your SO parent.. their older child.. their younger one?

What are you exactly expected to cope with.. and why hasn't your spouse been able to make the situation better?

Is it just that you don't like him having another child with another woman? just the inconvenience of having some other kid in your home?

Or... are there real unparented behavioral issues.

Counseling could help you cope.. but if the issues are external to you... and not being addressed (assuming they would be terrible behavior by the Skid).. then counseling can only take it so far for you.

is it worth staying?

Disengage,,,, As in don't see her...don't be around her. Or have her around your chid.. This may take a little effort on your part.  Have SO deal with her outside your home.  Or let SO pay for a hotel room for you and your child when SD is around.   Let SO know he must do something beside having his head in the sand 
...

I hope that you find this to be a good place to vent, contribute, and to pick up some useful perspective from others who are living the adventure of the blended family dream.

Whether the frustration is the SKids, the partner, their X(s), the ILs, or some or all of the these and more the SParent life struggle is real.  Never allow yourself to be sacrificed on the altar of SParental martyrdom to a mate, their failed family progeny, their X, etc. Not once, and for sure not repeatedly.

Let it out and share.  It helps.

If you have not implemented firm and enforced standards of behavior and standards of performance regarding the kids, your mate, your ILs, etc, you are missing a keystone of living well as a SParent. I advise that you invoke those immediately and defend them as a hill to die on.

Take care of  you.

Sincere regards,

Rags

Not everyone realizes this, but you get to choose. You can decide to disengage from all things SD, to step back and be just the wife. It's okay to prioritize your own mental health and just focus on your bio child, your self, and your marriage.

Fill your life up with things that benefit your bio child and you. The more you do that, the less head space you'll devote to other people's kids and other people's problems.

I feel you, MJay. I've been feeling the same way. I get so triggered when 19 yo SS stays with us. I've managed to express to dh that I simply do not want a 19 yo as a roommate. And dh fully admits that SS is loud, takes up all the air in the room, and is super clingy even still with dh. Dh admitted that he wouldn't want to have my DS as a roommate who is 21 yo (he does not live with us). I told dh 'what if I told you that you don't have a say, and you better just deal with it." That's essentially what dh tells me. If so, the least he could do is to make absolutely sure that I'm comfortable instead of yell at me whenever SS expresses dissatisfaction.

Stress and anxiety are mortally unhealthy. Just read the research. I do not want any of that, yet I still feel that SS has the right to stay here part time being dh's child. So I guess I just have to live with being swamped with anxiety every weekend. I have decided however that I will be staying away as much of the weekend as possible. That is upsetting dh, but that's the only solution that I can come up with.

Tell us what's been happening. Breathe. 

I am so sorry your situation has gotten this bad for you. Please tell us more.

Some have mentioned disengaging; while we do not have SKs living with us, I have almost completely disengaged from them. By 'them' I mean my DH's family, not just his adult kids. There is so much dysfunction and denial that I chose not be around it, as it is sick and unhealthy. (So many times I wonder how my DH emerged mostly unscathed from his family of origin?) Disengaging has helped me tremendously. When DH will bring someone up, I will give a pleasant response followed by an immediate change of subject, "Oh, that's nice. What should we have for dinner?" I realize this is not ideal, but I am playing the hand I was dealt to the best of my ability. It has been hard to see DH realize that his family is not who he thought they were. I understand because some members of my family struggle too.

Do you think disengaging could help you? I am also a proponent of counseling, it saved my health and sanity. Perhaps it could do the same for you?

Blessings to you, I care.